1. Field of the Invention
The present invention relates to interferometers for making highly accurate measurements of wavefront aberrations, particularly to phase-shifting point diffraction interferometers and rapid methods for in situ, object- and image-plane mask alignment in the interferometers.
2. State of the Art
Optical metrology is the characterization of surfaces and materials using optical methods. An area of optical metrology relevant to the present invention is the use of an interferometer to measure the quality of a test optic, such as a single or multiple element mirror or lens system.
One important recent application of optical metrology is the testing of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. In pursuit of this goal, systems are confronted with the diffraction limit determined in part by the wavelength of the light employed. To meet the challenge of imaging ever smaller features, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and may eventually move to even shorter wavelengths such as extreme ultraviolet or to yet shorter X-ray radiation.
For the extreme case of X-ray lithography, a proximity method that does not require re-imaging optics is under development. In X-ray proximity lithography, feature sizes are considerably larger than the wavelength limit. However, reticles in this case are required to have feature sizes equal to the desired printed feature size, currently on the order of 0.1 microns and smaller. It is quite difficult and expensive to manufacture reticles having such small feature sizes. Additionally, radiation passing through the reticle's narrow slits and apertures still diverges despite the extremely short wavelength. Thus, the reticles must be placed very close to the wafer, sometimes as close as a few microns, so that the shadow-cast image of the reticle remains sharp on the wafer. These systems must be carefully designed such that the reticle never contacts the wafer, an event that could destroy the reticle.
Because of the difficulties posed by proximity imaging a reticle pattern onto a wafer, it is desirable to extend the concepts of projection optics as currently used in visible-light or deep-ultraviolet lithography systems to even shorter wavelengths such as extreme ultraviolet. Such systems employ lenses or other optical elements to project a demagnified image of the reticle onto the wafer surface. This allows reticles to retain larger feature sizes, thus reducing the expense of generating the reticle itself
As with all optical imaging systems, various aberrations such as spherical aberration, astigmatism, and coma may be present. These aberrations must be identified and removed during the fabrication and/or alignment of the projection optics, or the projection optics would introduce substantial blurring in the image projected onto the wafer.
In order to test the projection optics for various aberrations, interferometers may be employed. Conventional interferometers, based upon the Michelson design for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram, and the resultant wavefront with, for example, Zernike polynomials, indicates the presence of aberrations.
The reference wave of the interferometer should be "perfect"; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optical elements through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical elements through which the reference wave passes. While these artifacts, in theory, can be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly clean interferogram.
To address this problem, "point diffraction interferometers" have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer (PS/PDI) described in H. Medecki, et al., "Phase-Shifting Point Diffraction Interferometer", Optics Letters, 21(19), 1526-28 (1996), E. Tejnil, et al., "At-Wavelength Interferometry for EUV Lithography," J. Vacuum Science & Tech. B, 15, 2455-2461 (1997), K. A. Goldberg, et al., "Characterization of an EUV Schwarzchild Objective Using Phase-Shifting Point Diffraction Interferometry," Proceeding SPIE, 3048, 264-270 (1997), E. Tejnil, et al., "Phase-Shifting Point Diffraction Interferometry for At-Wavelength Testing of Lithographic Optics," OSA Trends in Optics and Photonics: Extreme Ultraviolet Lithography, Optical Society of America, Wash. D.C., 4, 118-123 (1996), K. A. Goldberg, "Extreme Ultraviolet Interferometry", doctoral dissertation, Dept. of Physics, Univ. of California, Berkeley (1997), and in the U.S. Pat. No. 5,835,217 "Phase-Shifting Point Diffraction Interferometer", Inventor Hector Medecki, which are all incorporated herein by reference.
The PS/PDI is a variation of the conventional point diffraction interferometer in which a transmission grating has been added to greatly improve the optical throughput of the system and add phase-shifting capability. In the PS/PDI, as illustrated in FIG. 1A, the optical system 2 under test is illuminated by a spherical wave 5 that is generated by an entrance pinhole 6 in a mask 4 that is placed in the object plane of the optical system 2. To assure the quality of the spherical wave illumination, pinhole 6 is chosen to be several times smaller than the resolution limit of the optical system. Grating 8 splits the illuminating beam 5 to create the required test and reference beams 10 and 12, respectively. A PS/PDI mask 20 is placed in the image plane of the optical system 2 to block the unwanted diffracted orders generated by the grating 8 and to spatially filter the reference beam 12 using a reference pinhole 16. The test beam 10, which contains the aberrations imparted by the optical system, is largely undisturbed by the image plane mask by virtue of it passing through a window 14 in the PS/PDI mask 20 that is large relative to the point-spread function of the optical system. The test and reference beams propagate to the mixing plane where they overlap to create an interference pattern recorded on a CCD detector 18. The recorded interferogram yields information on the deviation of the test beam from the reference beam which in the ideal case is a spherical wave. FIG. 1B depicts a PS/PDI mask 20 comprising a square shaped window 14 and reference pinholes 21 and 24 and that are placed at 90 degrees for measurement in two orthogonal directions. The window also includes a symmetry-breaking feature in the lower left corner of the window to identify the corner opposite the reference pinholes.
The accuracy of the PS/PDI measurement system comes from the pinhole-diffraction-generated reference and illumination beams. This type of high-accuracy interferometer can be implemented in any spectral regime.
In practice, component alignment is the most challenging aspect of using the PS/PDI. Measuring high-resolution optical systems interferometrically requires the use of spatial-filter pinholes that are smaller than the diffraction-limited resolution. For example, accurate measurement of an advanced lithographic optical system, such as an extreme ultraviolet (EUV) projection system operating near 13-nm wavelength, requires pinholes below 100-nm in diameter. Alignment of these pinholes necessitates the use of translation stages capable of controlled motion on the same size scale.
In order to facilitate this most difficult aspect of measurement, and in order to incorporate a range of suitably-sized pinholes into the image-lane, a pinhole mask has been created that consists of an array of pinhole and window fields arranged in a rectangular grid, and spaced with sufficient clearance to allow the use of one individual field at a time. The existence of the pinhole array enables the optimal pinhole diameter to be selected in situ. A variant of this design, configured for the PS/PDI with square windows adjacent to each pinhole has been used in the measurement of prototype EUV lithographic imaging systems.
While the introduction of the pinhole array has many practical advantages, confusion frequently occurs regarding which of the field points is being illuminated. Intentional or accidental adjustment of the illumination source, the object pinhole, or the optical system can cause the interferometer-user to lose track of the position of the focused beam in the image-plane, and therefore also lose track of the beam's position within the pinhole array. The great similarity among the pinhole fields in the array contributes to this difficulty. When the position (or knowledge thereof) is lost, one method of re-establishing the alignment is to systematically translate the array (or the illuminating beam) from field to field, while counting the rows and columns until the corner is reached. Although this method does work, it is very time-consuming.